Gas-Phase Deposition of Aminopropylalkoxysilanes on Porous Silica

Abstract

Modification of porous silica with aminopropylalkoxysilanes was studied under an inert atmosphere by a gas-phase technique, atomic layer deposition. Trifunctional γ-aminopropyltrimethoxysilane and γ-aminopropyltriethoxysilane (APTS), bifunctional γ-aminopropyldiethoxymethylsilane (APDMS), and monofunctional γ-aminopropyldimethylethoxysilane were used as precursors to deposit surface-saturated molecular layers onto dehydroxylated silica surfaces. A silica bed was saturated with each of the vaporized aminosilanes studied using a reaction temperature of 150 °C and a pressure of 20−50 mbar. At higher reaction temperatures, viz., 280−300 °C, decomposition of aminosilanes was observed on the surface. Aminosilanes were observed to interact with the silica surface through a site adsorption mechanism. Elemental analyses and DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) were used for the characterization of aminosilylated silica samples. The pretreatment temperature of silica, 200−800 °C, and the precursor used were observed to affect the surface density and the bonding mode of aminosilanes on the surface. The surface densities of amino groups on silica decreased when the pretreatment temperature of silica was increased. Tri- and bifunctional aminosilanes were bound onto the silica surface through fewer alkoxy groups when the heat-treatment temperature of silica was increased. The most dense molecular layers were achieved with APDMS and APTS (2.0−2.1 molecules/nm2 on silica heat-treated at 200 °C) even though the differences between the precursors were not large. A linear correlation between the surface densities of amino groups and the number of isolated silanol groups on silica was observed.